In the field of devices for measuring a nonreciprocal effect in general and in the field of fiber-optic gyrometers in particular, one prior art document consists of an article incorporated by reference into the present patent application. This article, entitled “Gyromètre à fibre optique: principes et technologies” [Fiber-optic gyrometer: principles and technologies] authored by H. J. Arditty, Ph. Graindorge and H. C. Lefevre from the THOMSON-CSF Central Research Laboratory, was published in Revue technique THOMSON-CSF, Vol. 15, No. 3, pages 777 to 807, in September 1983. This article describes in particular a “minimal” configuration of a fiber-optic gyrometer and gives a few illustrative examples thereof. This minimal fiber-optic gyrometer configuration is based on the use of a phase modulator placed in one of the branches of the Sagnac ring of the fiber-optic gyrometer.
More precisely, the fiber-optic gyrometer of this prior art comprises several optical components among which are: a light source, a light detector, a monomode spatial filter, a Sagnac ring having two branches, two light splitters, said optical elements being placed so that, on the one hand, a first portion of the light emitted by the source can pass in succession through the first splitter, through the filter, through the second splitter, to enter the first branch of the ring and emerge via the second branch of the ring, pass through the second splitter, through the filter and through the first splitter and arrive at the detector, and on the other hand, a second portion of the light emitted by the source can pass in succession through the first splitter, through the filter, through the second splitter, to enter via the second branch of the ring and emerge via the first branch of the ring, pass through the second splitter, through the filter and through the first splitter and arrive at the detector, said optical elements being structured and placed in such a way that the light, that emanates from the source and arrives at the second splitter before having passed into the Sagnac ring, is polarized light. The second splitter is a semireflecting plate. The monomode spatial filter is polarizing, being formed by the combination of an actual monomode spatial filter and a polarizer.
More precisely still, FIG. 1 shows schematically an example of a fiber-optic gyrometer according to the prior art. The path of the light beams is indicated in the FIG. 1 by means of arrows. A laser light source 1 emits a laser beam. This laser beam arrives at a first light splitter 4, which is a semireflecting plate, only one portion of the light beam passing through the first light splitter 4 toward the polarizer 51. The light beam then passes through the polarizer 51 and emerges therefrom polarized in a given polarization direction. The light beam is then focused by a lens L onto an entrance of an optical fiber 52 constituting the actual monomode spatial filter. At the exit of the optical fiber 52, the light beam passes through a lens L before arriving at a second light splitter 6, which is a semireflecting plate. The laser beam is divided into two approximately equal portions, each portion being focused onto one of the entrances 81 or 82 of the Sagnac ring 8, which is a reel of optical fiber. The direction of circulation of the light from the branch 81 toward the branch 82 is called cw (standing for clockwise) and the direction of circulation of the light from the branch 82 toward the branch 81 is called ccw (standing for counterclockwise). The branch 82, for example, includes a phase modulator 9. After having emerged from the Sagnac ring 8, the two laser beam portions pass through the second splitter 6, only partly, to be superposed as a laser beam focused by a lens L onto one end of the optical fiber 52. On leaving the optical fiber 52, the laser beam passes through a lens L and then the polarizer 51, the combination of the optical fiber 52 and the polarizer 51 constituting a polarizing monomode spatial filter 5. One portion of the laser beam, which is now again polarized in the initial given polarization direction, is reflected by the first splitter 4 onto the detector 2. Thanks to the phase modulation effected by the phase modulator 9, the signal detected by the detector 2 is representative of the Sagnac effect, and therefore of the speed of rotation to which the fiber-optic gyrometer is subjected, and can be exploited.
One of the drawbacks that this prior art has is that the phase modulator 9 used in the fiber-optic gyrometer of this prior art is too complex and too expensive.